I have renovated my home several times, and each time that I use sand bricks, animals spawn in my house. All the doors are closed and I have a roof. Having all these animals show up gets quite annoying after a while. I have a link to a screenshot of this below. (If it's just my screwy iPod I apologize.)

At one point mobs were updated so that they only spawned on natural blocks.Any crafted blocks would prevent spawning.So, I guess Sand Bricks are still technically a natural block, hence why mobs spawn on them.It's too bad Apple is preventing Pixbits from exporting any more updates after May 1st; otherwise, I know they would fix this if it is in fact a bug.